A substantial amount of research is currently on-going in the field of optical micro-cavity physics, particularly directed at the ability to develop a high-Q cavity resonator. In general, resonant cavities that can store and re-circulate electromagnetic energy at optical frequencies have many useful applications, including high-precision spectroscopy, signal processing, sensing and filtering. Many difficulties present themselves when conventional planar (two-dimensional) technology, such as etching, is used to fabricate such resonant devices. In particular, the surfaces of such devices need to exhibit deviations on the order of a few nanometers to reduce scattering losses and conventional etching processes cannot routinely form such smoothness. Optical three-dimensional (3D) microcavity resonators, on the other hand, can have quality factors that are several orders of magnitude greater than typical surface etched, 2D resonators, since the microcavity can be shaped by natural surface tension forces during a liquid state fabrication step. The result is a clean, atomically smooth silica surface with low optical loss and negligible scattering.
Optical glass microcavity resonators have quality factors (Q) that are higher by several orders of magnitude than their electromagnetic counterparts. Measured Q's as large as 1010 have been reported for optical glass microcavities, whereas other types of optical resonators typically have Q's ranging from about 105 to about 107. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGMs) that are supported within the microcavities. For these structures, the high-Q light confinement is caused by total internal reflection from the surface of the cavity. Typically, for asymmetric dielectric cavities of this type, both in 2D and 3D, the high-Q modes are confined near a closed geodesic at the surface of the cavity or near a stable closed optical ray. At times, however, these devices suffer from the problem of overpopulation of the resonant modes, limiting the usefulness of such devices. Moreover, and from a more practical view, the ability to couple light into and out of such WGM devices is extremely difficult to achieve and almost impossible to replicate for manufacturability purposes.